The present invention relates to a radiating element operating in two separate bands or sub-bands with circular polarization, in the context of applications to radar or satellite telecommunications at microwave frequencies, for example.
In telecommunications, this type of radiating element is more particularly intended to be integrated into an antenna on board a satellite or on the ground to enable communication between the various entities of the system.
Using different frequency bands or different ranges of frequencies in the same band, such as the 20/30 GHz Ka band, for example, necessitates the use of radiating devices capable of operating over a very wide band.
This necessity for a relatively wide frequency band is even more obvious when the radiating element must transmit and receive in two different frequency sub-bands.
In this case it is important for the frequency sub-bands to be relatively far apart to prevent the transmit and receive signals interfering with each other.
Prior art radiating devices operating over a relatively wide band are bulky and therefore costly to fabricate and complicated to use.
What is more, because of its structure, that type of wide-band device has a relatively limited surface efficiency.
It has been necessary to develop radiating elements operating in several bands or in several sub-bands of the same frequency band. This is known in the art.
For example, European Patent Application 0 130 111 discloses a radar source capable of emitting at least two frequencies so as to have high resolution at a high frequency and long range at a low frequency, for example.
The radar source employs four waveguides surrounding a fifth waveguide.
The four peripheral waveguides operate in the Ku band centered on 16 GHz and the central waveguide operates in the X band centered on 10 GHz, for example.
However, that kind of device operates only with linear polarization, and circular polarization necessitates the addition of a hybrid coupler, which increases the size and cost of the device. Moreover, high-frequency hybrid couplers cause high losses in the circuit.
The above kind of prior art device also necessitates a bulky and complex feeder system to radiate correctly, which makes the overall size even larger and the cost even higher.
What is more, an antenna including the above kind of source is intended to operate with a ratio of 6 or more between the highest and lowest frequencies, which does not impose severe operating constraints because of the separation of the highest and lowest frequencies.
However, with a ratio between the highest and lowest frequencies in the range from 1.22 to 2, the above kind of antenna is not efficient because of interaction between the various parts of the antenna.
So-called xe2x80x9cplanexe2x80x9d antennas employing integrated circuits and requiring no hybrid coupler are known in the art, in particular from French patent application 98 06200. However, plane antennas with a frequency ratio in the range from 1.2 to 2 are subject to high losses due to coupling of the elements operating in the high and low bands, in particular because of their compact size.
Against the above background, an object of the present invention is to alleviate the above drawbacks by proposing a compact low-loss dual band microwave radiating element and generating the circular polarization by means of the radiating part of the antenna itself, without requiring any additional circuit such as a hybrid coupler, for example.
To this end, in a microwave radiating element of the invention and including first and second means adapted to convey electromagnetic waves in respective first and second frequency bands, the first and second means are coaxial and the first means include a hollow metal waveguide adapted to receive the second means coaxially.
In a first embodiment the second means also include a hollow metal waveguide.
In a second embodiment the second means include a waveguide comprising a dielectric material core and a dielectric material covering and said dielectric waveguide is a microwave fiber which propagates only the H11 hybrid mode, for example.
In the first embodiment, the waveguides constituting the first and second means advantageously terminate in respective polarizers, the polarizers are interleaved one within the other and the geometry of the polarizers is such that electromagnetic waves are circularly polarized.
The polarizers are preferably of rectangular or elliptical cross-section.
In a preferred form of the second embodiment of the radiating element of the invention the geometry of the dielectric waveguide is such that electromagnetic waves are circularly polarized.
The core of the dielectric waveguide preferably includes an extension emerging from the covering of said waveguide and having elliptical, rectangular, or ellipsoidal cross-section.